Repeated sectioning of a material specimen on closely spaced parallel planes (commonly known as serial sectioning) is an established destructive technique for obtaining the 3-dimensional microstructure of a material specimen. Until now, the metallographic polishing techniques necessary to accomplish this task have been carried out manually, which is a time-consuming and technically high-risk process.
Existing methods for 3-D reconstruction of microstructures using serial sectioning rely on manual preparation of the sections, using a manual polishing machine and tripod polishing fixture. Hardness marking indents or other fiducial markings are used to measure the amount of material removed, and hence the thickness of the section. Once the required thickness is removed from the specimen, the specimen is then prepared for microscopy (electron or optical) and an image is taken of the surface. If large fields of view are required, many overlapping micrographs have to be taken and painstakingly reconstructed. This process is repeated for as many sections as are required. Estimates for the time taken to acquire each section range from 4 to 8 hours of skilled manual work, translating to many weeks and months of effort for any reasonable number of sections (between 50 and 100).
Other methods of serial sectioning include automatic micro-milling units that use precision diamond cutters, and also microtomes with glass or diamond blades, which can remove thin layers of material from the specimen. Although rapid, these methods have the disadvantage that only a limited number of different materials systems can be sectioned successfully using these techniques, and still produce a surface that is suitable for microscopic imaging.
In all of the current techniques, the measurement of distance between subsequent sections, plus the in-plane alignment between them is key in reconstructing the 3-D microstructure of the specimen. This means the painstaking repeated placement of fiducial markings on the constantly receding polished surface, and the measurement of the changes in their geometry, as sections are obtained. This is a highly time-consuming process and prone to error.
The invention described herein is an automatic method that accomplishes the same task as manual serial sectioning, but with a higher precision and repeatability, and a 100× increase in speed. This increase in speed is enabling in materials science research because it allows the 3-D reconstruction of material microstructures in a matter of hours, rather than the many weeks and months it currently takes. It is also enabling in a materials production environment, as 3-D microstructural data can be analyzed in real-time and the results applied toward optimizing the process, again in real time.